I'm currently trying to learn GO and mainly knowing and working with Java, ASP.Net and some Python, there is no experience working with C-like pointers, which causes my current confusion.
A library I'm currently using to write my first GO project is called Commando.
There I have the struct CommandRegistry and the variable of interest is called Commands.
In the struct the variable is described as the following:
// registered command configurations
Commands map[string]*Command
On a first glimpse I would understand this as a Map object containing a list of Strings, however it also shows the pointer reference to the actual Command object.
All I can see is that it is a map I can loop over which returns the name of the command ( the string ),
however I'm wondering if the *Command in the type description means I can somehow dereference the pointer and retrieve the object itself to extract the additional information of it.
As I know the & operand is used to create a new pointer of another object. Pass-by-reference basically instead of pass-by-value.
And the * operand generally signals the object is a pointer or used to require a pointer in a new function.
Is there a way I can retrieve the Command object or why does the type contain the *Command in it's declaration?
Commands is a map (dictionary) which has strings as keys, and pointers to Commands as values. By passing it a key, you will get a pointer to the command it belongs to. You can then dereference the pointer to an actual Command object by using the * operator. Something like dereferencedCommand := *Commands["key"].
The * operator can be quite confusing, at least it was for me. When used as a type it denotes that we are receiving the memory address of some variable. But to dereference a memory address to a concrete type, you also use the * operator.
i am new to the Esp Idf and a beginner programmer and I would like to initialize the adc adc_channel_t via a pointer string but I get an error "conflicting type qualifiers"
can someone help me how to initialize adc_channel_t via own variable so that I can pass in that variable via a own function wehre I can change the channel as argument.
this is the code
const char *ptr ="ADC_CHANNEL_0";
static const adc_channel_t ptr;
adc1_config_channel_atten(ptr, atten);
Thanks
The documentation for adc1_config_channel_atten() can be found in the ESP IDF programming guide.
First of all, you are declaring the ptr variable twice with two different types. That doesn't work. You can declare the variable only once.
Second, you'll see from the documentation that adc1_config_channel_atten expects the enum type adc1_channel_t as type of the first parameter. You can not pass a const char* there. If you have to use a string for whatever reason, you need to write a custom conversion function which takes the string as an argument and returns a variable of the type adc1_channel_t (or an error value, if something can go wrong).
Hope this helps! Let us know in the comments if something is unclear.
In the plan9 specific Go code for syscall, there is no Stat_t like with other GOOS. Where is Stat_t, or its equivalent defined?
TL;DR: It's the *syscall.Dir type. Read on for details.
The source for os.Stat on Plan9 is here. It calls dirstat, which is defined here. It feeds the return value of dirstat into fileInfoFromStat, which is defined in the same file here.
In the case of paths (as opposed to *File objects), dirstat just calls syscall.Stat, which is basically just a thin wrapper around stat. syscall.Stat expects a byte buffer to be able to write into. This buffer is processed a bit (see dirstat for details), and then fed into syscall.UnmarshalDir, which is where the magic happens. The documentation states that it "decodes a single 9P stat message" from a buffer and returns a *syscall.Dir.
dirstat then passes this *syscall.Dir to fileInfoFromStat, which is what processes it into a FileInfo. It's this *syscall.Dir value that is obtained through the Sys() method on the FileInfo object.
If we open some arbitrary file using fopen() we get a pointer to it and it seems on windows, it will be stored in varying address (for example 1fb8e50)
So can I declare a second pointer to it, from another instance like that:
volatile FILE* fp = (volatile FILE*)0x1fb8e50;
And use them both. Is that dangerous or just possible for that matter?
Something like this (yes, this doesn't deal with some edge cases - that's not the point):
int CountDigits(int num) {
int count = 1;
while (num >= 10) {
count++;
num /= 10;
}
return count;
}
What's your opinion about this? That is, using function arguments as local variables.
Both are placed on the stack, and pretty much identical performance wise, I'm wondering about the best-practices aspects of this.
I feel like an idiot when I add an additional and quite redundant line to that function consisting of int numCopy = num, however it does bug me.
What do you think? Should this be avoided?
As a general rule, I wouldn't use a function parameter as a local processing variable, i.e. I treat function parameters as read-only.
In my mind, intuitively understandabie code is paramount for maintainability, and modifying a function parameter to use as a local processing variable tends to run counter to that goal. I have come to expect that a parameter will have the same value in the middle and bottom of a method as it does at the top. Plus, an aptly-named local processing variable may improve understandability.
Still, as #Stewart says, this rule is more or less important depending on the length and complexity of the function. For short simple functions like the one you show, simply using the parameter itself may be easier to understand than introducing a new local variable (very subjective).
Nevertheless, if I were to write something as simple as countDigits(), I'd tend to use a remainingBalance local processing variable in lieu of modifying the num parameter as part of local processing - just seems clearer to me.
Sometimes, I will modify a local parameter at the beginning of a method to normalize the parameter:
void saveName(String name) {
name = (name != null ? name.trim() : "");
...
}
I rationalize that this is okay because:
a. it is easy to see at the top of the method,
b. the parameter maintains its the original conceptual intent, and
c. the parameter is stable for the rest of the method
Then again, half the time, I'm just as apt to use a local variable anyway, just to get a couple of extra finals in there (okay, that's a bad reason, but I like final):
void saveName(final String name) {
final String normalizedName = (name != null ? name.trim() : "");
...
}
If, 99% of the time, the code leaves function parameters unmodified (i.e. mutating parameters are unintuitive or unexpected for this code base) , then, during that other 1% of the time, dropping a quick comment about a mutating parameter at the top of a long/complex function could be a big boon to understandability:
int CountDigits(int num) {
// num is consumed
int count = 1;
while (num >= 10) {
count++;
num /= 10;
}
return count;
}
P.S. :-)
parameters vs arguments
http://en.wikipedia.org/wiki/Parameter_(computer_science)#Parameters_and_arguments
These two terms are sometimes loosely used interchangeably; in particular, "argument" is sometimes used in place of "parameter". Nevertheless, there is a difference. Properly, parameters appear in procedure definitions; arguments appear in procedure calls.
So,
int foo(int bar)
bar is a parameter.
int x = 5
int y = foo(x)
The value of x is the argument for the bar parameter.
It always feels a little funny to me when I do this, but that's not really a good reason to avoid it.
One reason you might potentially want to avoid it is for debugging purposes. Being able to tell the difference between "scratchpad" variables and the input to the function can be very useful when you're halfway through debugging.
I can't say it's something that comes up very often in my experience - and often you can find that it's worth introducing another variable just for the sake of having a different name, but if the code which is otherwise cleanest ends up changing the value of the variable, then so be it.
One situation where this can come up and be entirely reasonable is where you've got some value meaning "use the default" (typically a null reference in a language like Java or C#). In that case I think it's entirely reasonable to modify the value of the parameter to the "real" default value. This is particularly useful in C# 4 where you can have optional parameters, but the default value has to be a constant:
For example:
public static void WriteText(string file, string text, Encoding encoding = null)
{
// Null means "use the default" which we would document to be UTF-8
encoding = encoding ?? Encoding.UTF8;
// Rest of code here
}
About C and C++:
My opinion is that using the parameter as a local variable of the function is fine because it is a local variable already. Why then not use it as such?
I feel silly too when copying the parameter into a new local variable just to have a modifiable variable to work with.
But I think this is pretty much a personal opinion. Do it as you like. If you feel sill copying the parameter just because of this, it indicates your personality doesn't like it and then you shouldn't do it.
If I don't need a copy of the original value, I don't declare a new variable.
IMO I don't think mutating the parameter values is a bad practice in general,
it depends on how you're going to use it in your code.
My team coding standard recommends against this because it can get out of hand. To my mind for a function like the one you show, it doesn't hurt because everyone can see what is going on. The problem is that with time functions get longer, and they get bug fixes in them. As soon as a function is more than one screen full of code, this starts to get confusing which is why our coding standard bans it.
The compiler ought to be able to get rid of the redundant variable quite easily, so it has no efficiency impact. It is probably just between you and your code reviewer whether this is OK or not.
I would generally not change the parameter value within the function. If at some point later in the function you need to refer to the original value, you still have it. in your simple case, there is no problem, but if you add more code later, you may refer to 'num' without realizing it has been changed.
The code needs to be as self sufficient as possible. What I mean by that is you now have a dependency on what is being passed in as part of your algorithm. If another member of your team decides to change this to a pass by reference then you might have big problems.
The best practice is definitely to copy the inbound parameters if you expect them to be immutable.
I typically don't modify function parameters, unless they're pointers, in which case I might alter the value that's pointed to.
I think the best-practices of this varies by language. For example, in Perl you can localize any variable or even part of a variable to a local scope, so that changing it in that scope will not have any affect outside of it:
sub my_function
{
my ($arg1, $arg2) = #_; # get the local variables off the stack
local $arg1; # changing $arg1 here will not be visible outside this scope
$arg1++;
local $arg2->{key1}; # only the key1 portion of the hashref referenced by $arg2 is localized
$arg2->{key1}->{key2} = 'foo'; # this change is not visible outside the function
}
Occasionally I have been bitten by forgetting to localize a data structure that was passed by reference to a function, that I changed inside the function. Conversely, I have also returned a data structure as a function result that was shared among multiple systems and the caller then proceeded to change the data by mistake, affecting these other systems in a difficult-to-trace problem usually called action at a distance. The best thing to do here would be to make a clone of the data before returning it*, or make it read-only**.
* In Perl, see the function dclone() in the built-in Storable module.
** In Perl, see lock_hash() or lock_hash_ref() in the built-in Hash::Util module).